Copper

Cu is a relatively common trace element in nature and the 28th most frequently found in the earth crust. It shows often elevated surface water levels in agricultural areas (see section 5.5.7). Data obtained between 1980 and 1992 indicate for Scandinavian surface waters (lakes) a total or dissolved median concentration below 1 |g/l, whereas > 97% of the lakes had less than 3 |g/l, which is, using the LBRL levels used in Lydersen et al. (2002), supposed to be the threshold value for biological effects in sensitive waters. Cu concentrations in 126 uncontaminated groundwater aquifers in Sweden sampled in 1985 to 1987 varied between 0.05 and 9 |g/l. High TOC lakes (> 10 mg/l) show the highest (median) Cu concentrations (0.56 |g/l) in contrast to low TOC lakes (< 5 mg/l) with median concentrations of 0.31 | g/l. pH seems not to be so relevant in determining Cu concentrations in Scandinavian lake waters. Cu concentrations in soils generally range from 10 to 100 mg/kg dw depending on the mother material. A survey of Swedish forest soils with 360 samples from a mor layer (see Lydersen et al. 2002) showed median Cu concentrations in the range 3 to 36 mg/kg dw, whereas Cu in Norwegian soils is about 10 mg/kg dw in the humus layer. In sediments, Cu is efficiently retained from uptake by organisms due to the formation of stable complexes with negatively charged organic compounds. While Cu adsorption to Fe and Mn oxides and oxyhydroxides, and to organic matter, may be an efficient retention process in oxidic sediments, precipitation of Cu sulphide may constitute a major sink in anoxic sediments. Based on sediment analyses from relatively uncontaminated lakes, sediment Cu concentrations in Sweden are supposed to vary between 16 to 38 mg/kg dw (south), 10 to 34 mg/kg dw (central) and 7 to 27 mg/kg dw (north). In other European countries the measured or estimated background copper concentrations in freshwater sediments range between about 2 and 60 mg/kg dw.

Metallic copper Cu0 is chemically rather inert and dissolves very slowly in oxic and slightly acidic environments. The divalent form Cu2+ is the normal valence state in oxic, aqueous environments and many of the divalent Cu compounds, such as Cu sulfate, are readily soluble in water. However, cupper oxide (pK = 20.5) and copper pyrite (pK = 97.6) are very insoluble in water. The Cu2+ ion forms strong complexes with hydroxide and organic ligands, e. g. humic and fulvic acids. Various biomolecules, such as sulfur-containing amino acids and proteins (e. g. metallothionein), can form extremely strong Cu(II) complexes and are used by organisms to regulate their internal copper level. Cu2+ is also readily adsorbed by clay particles.

Most of the different selectivity observed for metals to form complexes with ligands might be explained by ionic charcteristics such as polarizability, hydration conditions, and the ability to form stable organo-metallic complexes. However, Cu2+ is an exception from this rule. According to the polarizability and hydratisation energy, the degree of selectivity to organic complexes for divalents metals should be as follows: Pb > Cd > Co > Zn ~ Ni >Cu. However, Cu exhibits the strongest assymetry in coordinate positions around the Cu2+ ion, meaning that two ligands have larger binding lengths (Phillips and Williams, 1996; cited in Lydersen et al. 2002). Thus H2O molecules associated in these positions are far easier to remove, explaining the strong binding between Cu and organic ligands. As a consequence, the order of bonding strength between metals becomes reversed. Based on a review of factors that are most essential for bonding strength, the usual divalent cations may be ranked as follows: Hg = Pb > Cu >> Cd > Ni > Co > Zn > Mn > Ca > Mg. Focusing on the metals that are actual for this update, Cu turns out to have the strongest association to organic compounds. Metals with a medium bonding strength to organics include Ni and Zn, while Ca and Mg exhibit the weakest bonding strength.

pK1 values for water- or soil-derived fulvic acids have been reported to be in the range 5.5 - 7.0 and pK2 values in the range 3.8 - 5.4 (Borg and Johansson, 1989; cited in Lydersen et al. 2002), which agrees well with values found for Cu organic complexes in water of 10 lakes in northern Sweden (pK1 = 5.4 - 6.4; pK2 = 4.2 - 5.0). The potential Cu binding capacity of the organic compounds in these waters was in the range 64 -2,200 ^g Cu/l (Borg and Johansson, 1989), which is extremely high in comparison with total concentrations of copper found in Scandinavian freshwaters (Lyderssen et al., 2002).

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